1. Field of the Invention
The present invention relates to polarization diversity in an antenna system and, more particularly, to a polarization diversity antenna which has a simple structure and a small size.
2. Description of the Related Art
In the antenna field, polarization means a polarity direction of an E field with respect to a propagation direction of an electromagnetic wave. Every antenna has polarization of its own, and matching of polarization directions of transmitting and receiving antennas is an important consideration. The polarization can be classified into linear polarization and circular polarization.
Polarization diversity is a technology for improving frequency efficiency in mobile communications using different frequencies of adjacent cell base stations. In this technology, two frequency signals are cross-polarized using a single antenna.
That is to say, two frequency signals which do not interfere with each other and have an orthogonal phase are mixed to be used for the single antenna. In this manner, the same frequency can be reused in the neighboring cell, thus enhancing user capacity.
In related art, a dual-polarization antenna or a mechanically rotating feed line is used to realize the above-mentioned polarization diversity.
However, the former is problematic in that a structure for achieving polarization diversity is very complicated and a large amount of power is consumed, and the latter is problematic in that reliability is reduced due to mechanical breakdown.
U.S. Pat. No. 5,977,929 discloses a structure of a polarization diversity antenna which is shown in FIG. 1.
Referring to FIG. 1, a crossed-dipole antenna includes four antenna elements 12, 14, 16, and 18, and a switching circuit 40.
The switching circuit 40 controls operation of the antenna elements 12, 14, 16, and 18 so as to provide vertical linear polarization and horizontal linear polarization, and acts as a radio frequency (RF) switching element having a plurality of PIN diodes.
Further, the switching circuit 40 has a voltage source 42 for providing direct current (DC) voltage to the switching circuit 40, a pair of DC blocking capacitors C1 and C2, and inductors L1, L2, and L3 blocking a radio frequency signal.
The capacitor C1 is connected to a positive RF signal input terminal 44 and the capacitor C2 is connected to a negative RF signal input terminal 46 to block the DC voltage from the RF signal input terminals 44 and 46.
Capacitors C1 and C2 may have the same value.
In addition, the inductor L1 is connected to the voltage source 42 to block an RF signal from the voltage source 42, and the inductor L3 is connected to a ground to block the RF signal from ground.
If positive bias voltage is applied through the voltage source 42 to the switching circuit 40, PIN diodes D2 and D3 are turned on and PIN diodes D1 and D4 are turned off. Therefore, the RF signal flows through the PIN diodes D2 and D3 of the switching circuit 40 as indicated by arrows 48 in FIG. 1.
Hence, the antenna element 14 is coupled with the antenna element 16 and the antenna element 12 is coupled with the antenna element 18, so that the positive bias DC voltage applied to the switching circuit 40 forms horizontal linear polarization moving from a left side to a right side in FIG. 1.
On the other hand, if negative bias voltage is applied through the voltage source 42 to the switching circuit 40, the PIN diodes D1 and D4 are turned on and the PIN diodes D2 and D3 are turned off. Therefore, the RF signal flows through the PIN diodes D1 and D4 of the switching circuit 40 as indicated by arrows 50 in FIG. 1.
Accordingly, the antenna element 12 is coupled with the antenna element 14 and the antenna element 16 is coupled with the antenna element 18, so that negative bias DC voltage applied to the switching circuit 40 forms vertical linear polarization moving from a lower side to an upper side in FIG. 1.
A terminal of the inductor L2 is connected to anodes of the PIN diodes D1 and D3, and another terminal is connected to cathodes of the PIN diodes D2 and D4. When a bias current is transmitted through the inductor L2, the inductor L2 prevents the RF signal from flowing.
+Vrf which is applied to the terminal 44 and −Vrf which is applied to the terminal 46 denote an RF driving signal for the switching circuit 40. In connection with this, −Vrf has a phase difference of 180° with respect to +Vrf.
The diversity antenna shown in FIG. 1 has a simpler and more efficient structure in comparison with a former antenna.
However, in the diversity antenna, it is necessary to use a bidirectional bias signal to control a switching circuit. This is not a desirable solution since most RF devices have a single unipolar power source. Furthermore, there is a problem in that the antenna cannot be operated without bias voltage.